A conventional, ordinary solar cell element is provided with a silicon semiconductor substrate, an n-type diffusion layer, an antireflective film, a rear surface electrode, and a front surface electrode. When forming the front surface electrode, an electrode pattern is formed by screen printing, stencil printing or the like using a conductive paste made by mixing conductive particles composed mainly of silver with glass frit, an organic vehicle, and the like. Thereafter, this electrode pattern is fired to form the electrode.
The increased environmental awareness in recent years has led to a desire for switching to lead-free materials and parts in solar cells.
Examples of lead-free glass include the zinc borosilicate glass frit described in Patent Document 1, the bismuth borosilicate glass frit and zinc borosilicate glass frit described in Patent Document 2, the borosilicate glass frit described in Patent Document 3, and the zinc borate glass frit described in Patent Document 4.
On the other hand, as an example of glass that can be fired at low temperatures, tellurium-based glass is known for use in fluorescent display tube sealing applications (Patent Document 5) and optical fiber material applications (Patent Document 6). However, the field of application of such tellurium-based glass is limited, and the use of tellurium-based glass in the formation of a conductor was rarely taken into consideration in the past. In addition, as a glass composition for a die-bonding adhesive, Patent Documents 7 and 8, for example, describe the use of tellurium-containing glass that can be fired at low temperature. Unfortunately, such glass contains a large amount of lead as an essential component, which is problematic in terms of environment and safety.
Focusing on such tellurium-based glass, the applicant of the present invention confirms that the use of a conductive paste containing tellurium-based glass to form an electrode of a solar cell element can lead to achievement of significant effects (Japanese Patent Application No. 2009-247220: referred to as “specification of the prior application,” hereinafter).
The present invention was contrived as a result of further research on this tellurium-based glass. In other words, although the conductive paste described in the abovementioned specification of the prior application uses silver as a conductive component, the inventors of the present invention had discovered that the excellent characteristics can be realized by a specific type of tellurium-based glass even if it contains copper or nickel as a conductive component, and thereby accomplished the present invention.